A battery takes out electrical energy by converting chemical energy to the electrical energy through a redox reaction at a cathode and an anode, or accumulates the electrical energy through a reverse reaction thereof. The battery has been used as a power source in various devices. Recently, due to rapid market growth of a device such as a laptop computer or a smartphone, a demand has been increasing for dramatically improving energy density and output density of a secondary battery used for the device. In order to alleviate the power shortage after the Great East Japan Earthquake, expectations have been rising for development of a large scale and large capacity secondary battery. For the purpose of meeting the above demands, there has been actively developed a battery that includes an alkali metal ion (e.g., a lithium ion) as a charge carrier and utilizes an electrochemical reaction in accompany with donating and accepting a charge by the charge carrier.
However, most of lithium ion batteries include an electrode material on a cathode side (cathode active material) having smaller discharge capacity (Ah/Kg) than an electrode material on an anode side (anode active material). This is a main reason why the lithium ion batteries cannot be increased in capacity. Lithium ion batteries currently available in the market include a metal oxide having high specific gravity as the cathode active material, which is problematic in terms of insufficient battery capacity per unit mass. Therefore, attempts have been made to develop a large capacity battery using a lighter electrode material.
For example, Patent documents 1 and 2 disclose a battery including an organic compound containing a disulfide bond as the cathode active material. In this battery, the disulfide bond is subjected to two-electron-reduction upon discharge to cleave a sulfide bond. The resultant products are reacted with metal ions in an electrolyte to turn into two metal thiolates. The two thiolates are subjected to two-electron-oxidation upon charge to turn back into sulfides. Thus, the battery operates as a secondary battery. This battery includes, as an electrode material, an organic compound mainly containing an element having low specific gravity (e.g., sulfur and carbon). Therefore, a certain effect can be achieved in that a large capacity battery having high energy density is made. However, there are problems of low re-bonding efficiency of cleaved disulfide bonds and unsatisfactory stability under a charge or discharge state.
Similarly, as batteries including an organic compound as an active material, Patent document 3 discloses a battery including a polypyrrole complex and Patent document 4 discloses a battery including a nitroxyl radical compound as the cathode active material. A piperidyl-group containing high molecular weight polymer or copolymer is described as the nitroxyl radical compound.
Non-patent document 1 discloses a secondary battery including 2,2,6,6-tetramethyl piperidinoxyl-7-yl methacrylate (PTMA) as the cathode active material.
However, a conductive polymer such as polypyrrole has a problem that generated charges spread throughout the polymer to cause strong Coulomb repulsion between the charges, so that the charges can be injected and emitted only in an amount equal to or smaller than a certain amount. The nitroxyl radical is advantageous in that a large current can be achieved due to rapid donation and acceptance of the charges at an electrode. However, the nitroxy radical is subjected to a redox reaction at a rate of 1 electron per 1 molecule, which is unsuitable for capacity enlargement of a secondary battery.
Meanwhile, Patent document 5 and Non-patent document 2 disclose a secondary battery including a low molecular weight organic compound having multi-step redox capacity. This battery has high capacity density, but includes the low molecular weight compound. Therefore, there is a problem that battery performance is deteriorated due to elusion to an electrolyte. There is a need to solve this problem.